The use is known, in common AC/DC conversion circuits of a certain power, of two stages connected together.
The first stage is the PFC (Power Factor Correction) at input, designed to take a current from the mains which is as sinusoidal as possible and in phase with the input voltage, so as to absorb the maximum active power without however requiring from the mains absorption peaks tied to electrolytic capacities.
The PFC generally supplies a constant direct voltage stabilized with the next stage.
The second stage consists of a DC/DC converter which, taking the voltage supplied by the PFC, supplies at output a direct voltage, variable or fixed according to the user's requests, at the same time achieving the necessary galvanic insulation between mains voltage and output.
The typical intrinsic structure of the PFC does not envisage galvanic insulation and, because of the very type adopted, cannot supply at output a voltage below the peak value of the rectified input voltage. Considering the rated European voltage of 230 Volt with relative tolerances generally equal to +15/−20%, it can be presumed that, with an input voltage of 230V+15%≈265V to which corresponds a peak value of 265×1.41≈374 Volt, the stabilized voltage which the PFC can supply cannot be below around 275 Volt ignoring the various losses in the circuit.
This is the reason why, in the aforementioned conversion circuits, the output voltage of the PFC is generally set between 275 and 280 Volts or close-enough values.
This way, the certainty exists of the correct operation of this stage in the entire range of input voltages, without, correspondingly, there being any appreciable changes in output voltage.
The DC/DC stage which is downstream of the PFC is therefore generally supplied with this fixed and stabilized voltage.
In modern circuits of the resonant type, maximum efficiency is achieved by making the stage work in a highly precise point called “point of resonance” which is closely tied by the input/output voltage ratio according to the sizing of the circuit itself.
In practice, it is certainly possible to also optimize efficiency at very high values if both input voltage and output voltage are fixed. Now, the input voltage is fixed and ensured by the PFC but, in the event of the output voltage having to vary, there is a drop in efficiency of the stage forced to operate out of resonance.
Considering in particular the battery chargers used for electric vehicles, the output voltage of these devices must provide a large variation interval according to whether the battery is down or fully charged.
It is therefore obvious that, without particular solutions, the performance of these devices will only be maximized around the point of resonance and penalized in all the other points relating to the various values of the output voltage.
A possible solution is therefore to vary the input voltage of the DC/DC converter so as to follow the variations required by the output as much as possible, thereby making the system always work around the point of resonance.